Recently, with a growth of demand for low-fuel consumption of automobiles, it has been desired to reduce rolling-resistance of tires. To satisfy the desire, it is demanded to develop a rubber material having lower exothermal properties (low hysteresis loss properties).
In the meantime, from the viewpoint of safety, it is demanded to develop a rubber material having not only excellent brake performance (wet skid resistance) on wet pavement but also excellent operation stability.
In addition, it is demanded to develop a rubber material having a good processability and providing a vulcanizate having a reinforcing effect with the help of a reinforcing filler and having excellent tensile properties, fuel saving properties and abrasion resistance.
When silica is used as the reinforcing filler in place of carbon black, which has conventionally been frequently used, a rubber composition excellent in balance between low hysteresis loss properties and wet skid resistance is known to be obtained.
However, the surface of silica is hydrophilic. Thus, when silica is used in combination with a highly hydrophobic diene rubber to prepare a composition, silica particles are mutually aggregated in the composition. As a result, good dispersibility of silica particles cannot be obtained. To overcome this problem, attempts have been made to enhance affinity for a silica surface by introducing a functional group interacting with the silica surface into a diene rubber, thereby improving dispersibility of silica in the composition and enhancing low hysteresis loss properties.
For example, Patent Literature 1 discloses a modified diene rubber obtained by reacting a modifier having a glycidylamino group with a polymer end. Patent Literature 2 discloses a modified diene rubber obtained by reacting glycidoxyalkoxysilane with a polymer end. Patent Literatures 3 to 6 each disclose a modified diene rubber obtained by reacting an alkoxysilane containing a nitrogen atom with a polymer end and a composition containing the modified diene rubber and silica. Patent Literature 7 discloses a modified diene rubber obtained by reacting a silicon atom-containing vinyl compound within a polymer chain and with an end thereof, and a composition containing the modified diene rubber and silica.
If a composition is prepared from a modified diene rubber according to any one of Patent Literatures 1 to 6 and silica and this composition or the composition according to Patent Literature 7 is vulcanized, a vulcanized rubber excellent in balance between low hysteresis loss properties and wet skid resistance can be obtained.
In the field of a rubber composition, various studies have conventionally been made on a method for modifying a butadiene polymer by introducing a functional group thereinto in order to improve affinity for a filler.
Examples of a specific method thereof include a method of introducing a functional group into an initiation end or termination end or into both of them by use of anion living polymerization (see, for example, Patent Literatures 8 to 10); a method of copolymerizing monomers having a functional group (see, for example, Patent Literature 11); and a method of introducing a functional group by reacting the functional group with a double bond of a main chain after polymerization (see, for example, Patent Literatures 12 and 13).